1. Field of the Invention
This invention relates generally to four-wheel steering (4WS) vehicles. More particularly, the present invention relates to a system and method for pivotally controlling vehicle rear wheels for use in combination with a front wheel steering mechanism.
2. Description of the Prior Art
As is well known, various attempts have long been made to improve controllability of automobiles in various driving conditions. The four-wheel steering system is one example which has resulted from such attempts.
Japanese Patent Publication No. 60-44186 discloses a four-wheel steering system for a vehicle wherein the rotation of the steering wheel is mechanically transmitted to a rear wheel steering mechanism for causing the rear wheels to make a same or opposite phase pivotal movement depending on the rotational angle of the steering wheel from a neutral rotational position. The term "same phase pivotal movement" means that the rear wheels are pivoted in the same direction as the front wheels, whereas the term "opposite phase pivotal movement" means that the rear wheels are pivoted in the opposite direction relative to the front wheels.
According to the above Japanese patent publication, more specifically, the vehicle rear wheels are caused to make a same phase pivotal movement when the steering wheel is rotated from the neutral position within a predetermined small angular range. On the other hand, when the steering wheel is rotated beyond the predetermined range, the rear wheels are caused to make an opposite phase pivotal movement. The same phase pivotal movement of the rear wheels is suitable for cornering or lane-shifting during medium- or high-speed driving because the rear wheels can bear a part of the side force (as required to counteract the centrifugal force or lateral acceleration force) from the ground to stabilize the vehicle. The opposite phase pivotal movement of the rear wheels is suitable for making a U-turn or guiding the vehicle into a garage at a low speed since the thus oriented rear wheels reduce the turning radius of the vehicle.
In any of conventional 4WS systems wherein the rear wheel steering mechanism is mechanically connected to the front wheel steering mechanism, the pivotal angle for the rear wheels is inevitably determined in dependence only on the rotational angle of the steering wheel. Thus, such a 4WS system cannot take acount of the vehicle driving speed, and therefore does not necessarily provide reliable vehicle controllability.
In actual driving, it can occur that the steering wheel be rotated to a relatively large degree even when running at a medium- or high-speed. If the rear wheels are pivoted in the opposite phase direction in this driving condition, a dangerous situation will result. Such a dangerous situation may occur with a 4WS system wherein the pivotal angle for the rear wheels is determined only by the rotational angle of the steering wheel.
Indeed, the lateral acceleration force (centrifugal force), hence the side force (centripetal force) required to counteract the lateral acceleration force, is a function of the vehicle speed and the turning radius (determined by the rotational angle of the steering angle). Thus, in pivotally controlling the rear wheels, it is necessary to take acount of the vehicle speed as well as the rotational angle of the steering wheel in order to improve the controllability of the vehicle in medium- or high-speed running particularly. Further, it is also important, in realizing ideal vehicle controllability, that the rear wheels bear a suitable portion of the total side force at an optimum timing relative to the start of rotating the steering wheel.
Japanese Patent Application Laid-open No. 60-67275 discloses a 4WS system wherein the rear wheel steering mechanism is electrically or electronically controlled by monitoring the vehicle speed and the rotational angle of the steering wheel for control purposes. The rear wheel steering mechanism is driven by a stepping motor which is assisted by a hydraulic cylinder device. The cylinder device is added because the stepping motor alone is very low in output torque and therefore not enough to suitably pivot the rear wheels against the side force acting to pivotally return the rear wheels.
The 4WS system of the above laid-open Japanese application utilizes the vehicle speed as well as the rotational angle of the steering wheel to determine the pivotal angle for the rear wheels. Theoretically, therefore, such a 4WS system seems to solve the problems mentioned hereinbefore. In reality, however, this 4WS system has the following problems.
First, the stepping motor, though easy to accurately control its rotational amount, is incapable of providing a high-speed rotation. Thus, the stepping motor fails to quickly pivot the rear wheels for adaptation to the constantly changing driving condition particularly in high-speed running. Second, the stepping motor, because of the low torque, inevitably requires a power assist means such as hydraulic cylinder device for pivoting the rear wheels. Thus, the overall 4WS system becomes costly and complicated.
It is conceivable to substitute a DC electric motor, which can be made to have a high torque and a high rotational speed, for the stepping motor of the 4WS system of the above laid-open Japanese patent application. However, such replacement gives rise to a new problem that the rear wheels cannot be accurately pivoted to a target pivotal position. The reason for this is that the DC motor has a tendency to overrun beyond a target rotational position (corresponding to the target pivotal position of the rear wheels) due to inertial rotation even if the pivotal angle of the rear wheels is monitored to interrupt the drive current to the motor upon reaching the target rotational position.